The present invention relates to an improvement in an analog signal processing apparatus in which an analog signal is converted to a digital signal which is subjected to predetermined processes and thereafter is converted back to an analog signal. This analog signal is then provided as the output.
Recently, what is called a digital television has been developed and put into practical use. In this digital television, an analog signal such as a video signal or an audio signal is not subjected to an analog process as it is, but is converted to a digital signal. This digital signal is then subjected to various kinds of predetermined processes. The digital signal which was subjected to predetermined processes is reconverted to an analog signal, so that the R, G and B video signals, audio signals or the like are obtained. Namely, as shown in FIG. 1, an analog input signal such as a video signal, audio signal or the like is input to an A/D converter 1. The A/D converter 1 converts the input analog signal to the digital signal. A digital signal processing circuit 3 receives the digital signal from the A/D converter 1 and performs various kinds of processes such as a Y signal process, separation of a chrominance signal, synchronizing separation, discrimination of an audio signal, and the like to this digital signal. The digital signal which was subjected to the foregoing various kinds of processes is input to a D/A converter 5. The D/A converter 5 converts the input digital signal to an analog signal. In this way, the processed analog output signal such as a video signal, audio signal or the like is derived.
As described above, in the case of temporarily converting the analog signal to the digital signal and performing the digital signal processes, the following advantages are obtained as compared with the analog signal process without any A/D conversion. (1) The characteristics become stable. (2) The number of parts can be reduced. (3) The adjusting steps can be decreased. (4) Reliability is improved due to the use of an IC.
Conventionally, to improve accuracy in an analog signal processing apparatus as shown in FIG. 1, the performances of the A/D converter 1, digital signal processing circuit 3 and D/A converter 5 are improved and the A/D converter 1 and the D/A converter 5 have high accuracies. In addition, the digital signal processing circuit 3 executes the processes on the assumption that no error is included in the digital signal which is output from the A/D converter 1.
However, in the conventional analog signal processing apparatus shown in FIG. 1, the following drawbacks occur. (1) The highly accurate A/D converter 1 and D/A converter 5 are extremely expensive. Thus, the analog signal processing apparatus using highly accurate A/D and D/A converters (namely, a highly accurate analog signal processing apparatus) is very expensive. (2) In general, it is difficult to realize improvements in both the accuracy and the converting speeds of the A/D and D/A converters. Usually, in the case where the accuracies of the D/A converter and A/D converter are improved their operating speeds are lowered. Therefore, this results in a slower operating speed of the overall analog signal processing apparatus. (3) Even if the accuracies of individual parts are improved, it is impossible to correct the conversion error, distortion or the like based on the offset voltage which is developed in the A/D converter on the input side, or based on the nonlinearity of the conversion function, or the like. Thus, these conversion errors and distortions are output as incorrect signals. (4) In the case of constituting a highly accurate A/D converter and D/A converter, the bipolar technology is superior to the MOS technology in the present technology. On the other hand, with respect to the digital signal processing circuit, the MOS technology is superior to the bipolar technology. Therefore, there is technical difficulty in making the overall apparatus as one chip.